The present invention relates to an arrangement for the control of the power transmission to at least one axle of a motor vehicle provided with a lockable differential.
With multi-axle high power motor vehicles or with such motor vehicles, in which one axle is loaded more strongly than another, a load change when driving through a curve leads frequently to a more or less strong unstable driving behavior. If, for example, the driver of the motor vehicle drives through a curve and then suddenly takes his foot off the drive pedal, a jerk in the drive connection which occurs as a result thereof, especially at the unloaded curve-inner wheel of the driving axle, may lead to the loss of the contact friction between this wheel and the road surface. In that case, generally only half the transverse guide force is then available at this axle so that with higher transverse accelerations this leads to a turning-in with rear driven or rear loaded four-wheel drive motor vehicles, respectively, to a pushing of the vehicle by way of the front wheels (leaving the curve in a tangential direction) with front-driven or front-loaded four-wheel driven vehicles.
It is known to utilize in motor vehicles lockable or automatically locking differentials for avoiding a loss of the contact friction of the driven wheels. The action thereof, however, only starts after a loss of the friction contact so that it is not possible to sufficiently counteract the described turning-in, respectively, leaving of the curve.
It is also known to improve the load-change behavior of a motor vehicle by constructive measures, for example, in that one takes care for a more balanced axle load distribution by relocation of aggregates or axle arrangement points or by costly axle constructions. This, however, leads frequently to space problems in the vehicle or deteriorates under certain circumstances the inherent steering behavior of the vehicle, i.e., the steering behavior of the vehicle itself, in such a manner that the vehicle is no longer controllable within the limit range, respectively, a certain self-stabilizing influence is no longer present as exist, for example, with a vehicle with an eccentrically displaced center of gravity.
It is therefore the object of the present invention to so further develop a motor vehicle of the aforementioned type that the load-change behavior when driving through curves, especially in the curve boundary range, is decisively improved.
The underlying problems are solved according to the present invention in that the differential is continuously controllable in its locking action by way of an adjusting member activated by a control apparatus, preferably equipped with a microprocessor, in dependence on operating and/or driving parameters, a power output of an internal combustion engine serves as controlling operating parameter, and the control apparatus acts upon the adjusting member up to a power output value lying within a power band available from the internal combustion engine with a first value of a control magnitude corresponding to a higher locking value and above this power output value with a second value of a control magnitude corresponding to a lower locking value.
The advantages of the present invention reside in the first instance in the improved load-change behavior when driving through curves and in that compared to a conventional control arrangement for lockable differentials, no additional signals must be detected and processed.
The use of locking differentials in motor vehicles that are adapted to be activated continuously in their locking action is known as such, for example, from the DE-PS 34 37 435 corresponding to U.S. Pat. No. 4,754,835, from the DE-PS 34 37 436 corresponding to U.S. Pat. No. 4,792,011 and from the DE-OS 36 08 059 corresponding to U.S. Ser. No. 24,068 filed Mar. 10, 1987 and from the article by H. Bott and M. Babtke, "Der Porsche Typ 959 -- Gruppe B -- ein besonderes Automobil" ["The Porsche Typ 959 -- Group B -- A Special Automobile"]--Part 2, ATZ 88 (1986), Volume 6, pages 353 to 356 and is described therein in detail.
It has now been found that the turning-in behavior of such a four-wheel drive automobile can be decisively improved during a load change in curves by a permanent partial activation (for example 20% of the locking action) of the lockable cross-differential. However, in order to avoid thereby excessive stresses in the differential at higher velocities, the activating value may be reduced dependent upon exceeding a power output threshold at, for example, 50% full load, to a 5% locking action without resulting thereby again in any negative influence on the turning-in behavior.
For the improvement of the steerability at lower velocity (parking ability) and low engine output, it may be additionally appropriate to completely cancel the locking action below the power output threshold, respectively, below, for example, 20 km/h and to raise the same to the higher activating value of 20%, respectively, the power-dependent predetermined activating value within a velocity range between, for example, 20 km/h and 50 km/h.